Ceramic block filters are well known in the prior art. As these devices have evolved and their use in radio communications products has increased, the electrical performance characteristics required of them have become increasingly more stringent.
In many radio communications applications, a ceramic bandpass filter may not have a passband cutoff characteristic that is sufficiently steep, particularly above the passband frequency. While most prior art ceramic bandpass filters have a relatively sharp cutoff frequency below the passband, most have a relatively mild cutoff characteristic above the passband. A sharper cut-off characteristic above the passband can be achieved by a so-called transmission zero above the passband. (A transmission zero in the transfer function of a filter that is a mathematical term representing a physical characteristic of the filter. A transmission zero is usually a combination of a capacitive element connected in parallel with an inductive element, which is positioned in the signal path such that it will block signals of a particular frequency and pass signals within the passband. A transmission zero above the passband is called herein a high-side transmission zero.) Since many communications applications could benefit from a ceramic bandpass filter having a so-called high side transmission zero, a ceramic block filter that has an increased attenuation above the bandpass for these applications would be an improvement over the prior art.
In the prior art, these so-called high side zeroes are accomplished principally by so-called top patterning or top loading. This top patterning is well known in the art and is essentially comprised of patterns of conductive material that surround the metallized holes of a block filter and in part capacitively couples signals to ground metallization lining the block filter, at some frequency. These patterns are accomplished principally by screen printing conductive material onto the top of the block. Top patterning is difficult to precisely control and does not lend itself readily to batch tuning, a tuning process whereby many block filters are preferably tuned in the manufacturing process, and not individually.
Accordingly, a ceramic block filter that has or accomplishes so-called high side transmission zeroes that increase attenuation of undesired signals above the passband frequency and one that avoids the prior art problems of top pattern tuning, would be an improvement over the prior art.